1. Field of the Invention
This invention relates to electrochemical devices which employ liquid electrolytes. More particularly, this invention relates to electrochemical cells which generate electricity, such as batteries, fuel cells, and electrolyzers. This invention further relates to a method and means for preventing fire and explosion hazards associated with such devices.
2. Description of Related Art
An electrochemical cell is a device in which a chemical reaction is used as a source of energy to generate an electric current. These devices generally comprise an anode electrode, a cathode electrode, and an electrolyte disposed between the two electrodes. Certain of these devices utilize ionic conduction in a liquid electrolyte, e.g. liquid solutions and molten salts, to generate the desired electric current.
In recent history, there have been a number of instances of fires and explosions caused by overheated storage batteries. Other electrochemical devices are also prone to such hazards. However, such hazards can be avoided by shutting down the electrochemical device before the device reaches a critical state, for example, a predetermined temperature. U.S. Patent Application Publication No. 2005/0260486 A1 teaches a battery having a safety element connected between its positive and negative electrodes, which safety element comprises material having a Metal-Insulator Transition (MIT) characteristic where the resistance abruptly drops at or above a certain temperature. The material senses the rise of battery temperature and lowers the charged state of the battery when the battery is exposed to an elevated temperature, the battery temperature rises due to external impact by pressure, nails or nippers, the battery temperature rises due to increases in ambient temperature, or the battery temperature rises due to overcharging. The MIT characteristic is indicated to be a material-specific characteristic which is presented only by materials including vanadium-based oxides, Ti2O3, or materials to which an element such as St, Ba, La, etc. is added, where resistance of the materials abruptly changes according to temperature. The change in resistance is indicated to be caused by a phase transition of a crystalline structure between a metal and an insulator. By use of materials having the disclosed MIT characteristic, the battery is turned into a stable discharged state at elevated temperatures.